Re: LM324 and crossover distortion



On 3/31/2011 3:55 PM, Jim Thompson wrote:
On Thu, 31 Mar 2011 12:23:24 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxxxxx> wrote:

On Thu, 31 Mar 2011 14:48:56 -0400, Bitrex
<bitrex@xxxxxxxxxxxxxxxxxxxxx> wrote:

On 3/31/2011 2:40 PM, Bitrex wrote:
On 3/31/2011 2:21 PM, ChrisQ wrote:
Jim Thompson wrote:

Even shifting crossover to a higher voltage can be an improvement
depending
on the kind of signals being processed. If the signals have a varying
amplitude
about 0V, crossover will bite you every "cycle". If the crossover
distortion is
shifted to some other voltage, preferably well towards the rail, it
will bite you much
less frequently for many signals. And in proportion to signal level,
the distortion
is reduced.

But I'm with John L, the 324 family sucks - get a better opamp. The
technique of shifting
the crossover can reduce distortion even with a better opamp, but
unless you're scrutinizing
every penny there are many better choices out there.

Cassiope,

Why don't you produce a list of "better" OpAmps and their cost per
OpAmp?

...Jim Thompson

The '324 must have been one of the first quad op amps around, mid
seventies ?, We were quite excited by it's arrival at the time. Turned
out to be too noisy for audio work, but plenty good enough for a myriad
of other applications. The little bit later Raytheon 4136's and similar
were better, but the 324 was still a major milestone in the art, imo.

Regards,

Chris

In my experiments the problem with the LM324 for audio is also its slew
rate - with a slew rate of 0.3 V/usec the maximum amplitude of a sine
wave at 20kHz would be about 1.5 volts before running into slew rate
limitations, and at the same amplitude a square wave will stop looking
like a square wave at a much lower frequency.


Sorry, I did the math wrong - that should be 15 volts P2P! But the
problem with square waves still occurs!

Neeerp !-) Keep trying...

S = Omega*VP

So VP = S/Omega

20kHz and 0.3V/us => VP = 2.387V, or Vp-p = 4.7746V

My very first OpAmp design, MC1530/31, circa 1963-64, had 6V/us
slew-rate :-) But it required external compensation.

And it's still being sold, 48 years later...

http://www.lansdale.com/part_search.php?search=MC1530&Go.x=0&Go.y=0

I've even had inquiries to re-design it on a modern process :-)

...Jim Thompson

Forgot another bragging point for the MC1530/31... sliding-class-A
output stage... no cross-over distortion ;-)

...Jim Thompson

I found your PDF on it here:

http://www.analog-innovations.com/SED/MC1530-TeachingExercise.pdf

I'm not sure how increasing the emitter areas of the output transistors implements the sliding class A.

Without any PNPs for current source loads it must have taken some intuition to arrange things so that everything canceled out and left the output at zero volts!
.